The present invention relates to a method and a device for reducing the influence of distortion which is created during frequency-generating and/or signal-processing in a Doppler radar system.
When an electrical signal is to be processed in a system, it is often a requirement that the signal-processing should be as free as possible from distortion. There are many different types of distortion which can occur when an electrical signal is being processed, among which there may be mentioned non-linear distortion, such as overtone formation and intermodulation, and in addition additive distortion, such as additive tones, in most cases caused by crosstalk. The effect of these different types of distortion is that distortion products of the overtone type and intermodulation products arise. Some examples of signal-processing are analog/digital conversion, amplification and frequency conversion. Non-linearities in individual components included in a system add up, and the demands placed on the components included in large systems are therefore stringent in order to make it possible to keep the total distortion through the whole system as low as possible.
Among the problems which may arise as a result of distortion, are false echoes in radar systems, also called ghost echoes. These can occur, for example, in an airborne radar system where echoes are received from the ground at the same time as echoes from aircraft. As a result of the distortion products which occur in signal-processing and frequency-generating, the ground echo gives rise not only to larger and real echoes, but also to a number of small, false echoes. This in turn has the effect that small but real echoes from aircraft cannot always be detected since a relatively high detection threshold is normally used in order to filter out the small, false echoes which have arisen on account of distortion in the radar system. Unless a higher detection threshold is used, it is not possible for the operator, or for further signal-processing, to distinguish between small but real echoes and false echoes. This in turn can lead to even greater problems for the operator.
U.S. Pat. No. 4,709,237 describes a device for eliminating low-frequency thermal noise which has arisen in transmitter amplifiers and receiver amplifiers of a radar. Unfortunately, the device does not solve the problems of the distortion which occurs in signal-generating and signal-processing means in which non-linearities, inter alia, cause overtones.